Kb nr30

[crackle]

I have a KB NR30 which is working well but there is too much bass, especially on FM where there seems to be too much for the 10 inch speaker to handle.
Please can you advise the best way to reduce the bass a little?
Audio circuits attached below.

Thanks
Mike

[Restoration73]

I suggest ;
Reduce values, of C36, R24, C33 (lf roll off)
Increase value of R21 (reduce negative feedback)

[valves4amps]

Hi Mike, I suggest reducing C33 from 0.05uf to 0.01uf or play with the value until it suits your taste.

Dave.

[Leon Crampin]

Reduce the value of C36, which would need replacing anyway. 4n7 would be a good starting value for a set with a large 'speaker and a highish grid leak resistor in the following stage. You could also adjust the feedback network to give more feedback (lower audio gain) at LF, by increasing the value of C34.

Leon.

2 crossed posts.

[Nuvistor]

The tapped V/C possibly increases bass at low volume, the loudness control, if it’s worse at low volume I would disconnect the connections to the tap.

[crackle]

Hi
Thanks all for your comments, there are some useful suggestions there and I have tried some of them.
Here is what I have found.
First I experimented with Leon's suggestion of increasing the value of C34 in the feedback circuit, this was easy to do whilst the set was working by bridging it with additional capacitors. I went up to 0.47uF and that did make a difference, but not by much, I left C34 at 0.1uF.

Next I reduced the value of C33 to .01uf as suggested by Dave, this made an improvement, but not quite enough.

Then I experimented with disconnecting the loudness tap on the volume as suggested by Frank. That had a different effect to what I expected. the volume increased significantly and the bass content reduced. So in order to reduce the volume back to how it was I replaced C47 with a link, that reduced the volume to a more easily adjustable level with less bass.

So as we have it now, C33 is reduced from 0.047uF to 0.01uF and C47 replaced with a wire link.
The audio is now more acceptable to my ears and can now be turned up to LOUD before the speaker diaphragm starts to flap and sound distorted..

Thanks for your help.

Mike

[Mike. Watterson]

I wonder is something else faulty to cause so much bass?

[dazzlevision]

Post 5 in my thread might be relevant: https://www.vintage-radio.net/forum/...d.php?t=100005

[crackle]

Hi Dazzlevision.
Thanks, I will take a look at the 220 ohm resistor, I don't think I checked that one. I am going to have to get the chassis out again for further checks.
That was a lot of work you put into the wave change switch, well done.
I have also learnt that my radio must be the NR30/1 as it is the 8 valve version, I do not think I realise that when I originally worked on it back in 2012, it is fitted with an ECC84 and a 6BW6, and I can confirm the FM band is quite lively.

Mike

[HamishBoxer]

Are all the existing caps in tone circuit ok?

[crackle]

Yes they have all been changed for the correct values, except the ones recently changed, as mentioned above.

Mike

[Mike. Watterson]

Amazing layout!
Certainly seems a deluxe set.

[Edward Huggins]

What a Beast of a Table Radio! Interestingly the circuit shows no form of filter for those Plessey 4" tweeters - unless I'm missing something?

[crackle]

What I cant believe is how short KB made the wiring to the EM34 magic eye. There is barely enough to be able to turn the eye so the 2 pairs of wings are aligned horizontally. There is no room to strip back and re-terminate if one gets frayed.
But at least the eye is nice and bright even in daylight, and it certainly helps find the best point of tune on AM and FM.


Mike

[ajgriff]

Presumably the Plessey HF units are working ok? Probably a daft question.

Alan

[crackle]

Yes both the tweeters are working.
I checked R18 the 220 ohm resistor connecting the earthy end ov the volume pot to ground and like Dazzlevision it was out of spec. It seemed to be measuring between 310 and 460 ohms depending on if I pulled or pushed on the wires.


Now it is replaced with a new 220 ohm resistor the radio seems to have very slightly more bass with the wire link shorting C47, so I have Experimented again and found a 0.1uF cap causes far less bass than the original 0.01uF, but also oddly slightly less bass than the wire link, so that cap is now functioning as intended but with a larger value.

Thanks for your help everyone.

Mike